Injection mold machines for making molded parts utilize two part molds, having mold cavities formed therein, that can be separated to remove the molded part. The polymer molding material is heated to a molten state and forced through a passageway consisting of the feed screw barrel, nozzle, sprue bushing, runner and gate and into the mold cavity. Once the mold cavity is filled with molten material it is cooled so that the molded part solidifies. The two-part mold is then opened and the molded part removed in the usual manner. This process requires that at some point along the passageway there be a cold/hot junction, one side of which the material solidifies along with the cooling part and the other side of which the material remains molten. This cold/hot junction is typically provided by means of a sprue bushing having an entry hole that forms a portion of the hot portion of the passageway and a smaller tapered hole in intersection with the entry hole that forms a portion of the cold portion of the passageway. This tapered hole leads into a runner portion of the passageway that leads directly to the mold cavity. The tapered hole is usually quite long because it must extend deeply into the mold to reach the mold cavity. After the molded part is cooled, it has attached thereto the material from the runner and the tapered hole. This extra amount of material is waste and is removed from the part and discarded. When molding relatively small parts, such as electrical connector housings, the amount of extra material can exceed the material used in the molded part, thereby increasing material costs substantially. Additionally, the cycle or recovery time of the mold, that is, the time it takes to operate the mold to make a molded part is partly affected by the time that it takes to inject the material into the mold cavity, the tapered hole, and the runner as well as the time that it takes to replace the melted material that has been used. In an effort to reduce the amount of waste material and to reduce the recovery time, so called, hot sprue bushings have been developed. In such bushings the cold/hot junction is relatively close to the runner leading to the mold cavity so that the tapered hole is relatively short, however, the entry hole is correspondingly longer. To prevent the material in the entry hole from solidifying when the part is cooled, the portion of the bushing through which the entry hole extends is heated by means of a separate electric heater that is wound around the periphery of the bushing. This, of course, makes the outer diameter of the sprue bushing considerably larger, requiring that the bushing be further from the mold cavity necessitating a longer runner. Therefore, what is saved by shortening the tapered hole is somewhat lost by having a longer runner.
What is needed is a heated sprue bushing having a relatively short tapered hole, that is small enough to be used closely adjacent the mold cavity so that the runner is relatively short, thereby using less material and shortening the recovery time.